It's the one with 4 logs and a pit in the middle. You have to put one up top and one at the bottom. It takes 3 logs to walk across the pit to the other side. Maybe it's number 7, I'm not sure as it doesn't say when I continue. There is one lone yellow block on the lower left side. You have to use a log to get a second log to its target, but I just don't see how to get across the pit to the end spot. If you go down into the pit, you can't get back out.